1. Field of the Invention
The present invention relates to a method for reducing a ripple, caused by rotational nonuniformity of an internal combustion engine, in the output voltage of a generator driven by the internal combustion engine.
2. Description of the Related Art
Claw pole generators having electrical excitation are usually used as generators in motor vehicles. The current through the rotor winding is used as a manipulated variable for regulating the desired output voltage or a requested torque, and is predefined by an associated field controller. This type of regulation is necessary, since otherwise, due to the highly variable engine speeds, greatly fluctuating voltage values would be supplied by the generator which could possibly damage the downstream electrical system.
Since the rotor winding typically has a time constant>150 ms, rapid control operations are not possible. However, in particular at low rotational speeds, in particular in the range of the idle speed, this results in voltage ripples, since in this case the power output of the generator is highly dependent on the rotational speed. If the rotational speed fluctuates very rapidly due to rotational nonuniformity in this range, the field controller is not able to respond quickly enough to the fluctuations on account of the rotor time constant of the rotor field, resulting in an undesirable ripple in the generator output voltage. This effect is further intensified in generators, which in addition to the rotor winding have permanent magnets for leakage flux compensation, since in this case the power characteristic curve has an even steeper slope.
Published European patent document EP 0 847 485 B1 discloses a method for damping torque fluctuations due to rotational nonuniformity of the internal combustion engine, a generator being used in motor mode to compensate for torque fluctuations. However, this further intensifies the voltage ripple.
It is therefore desirable to provide an option for reducing a ripple in the output voltage of a generator driven by the internal combustion engine.